This invention pertains to an all optical crossbar switch, in which each of multiple input sources is selectively connected optically to any of multiple output receivers.
In recent years increasingly large amounts of voice, video, and data have been carried by optical fibers. To minimize signal dispersion, these fibers generally support only a single optical mode each. To increase capacity large numbers of fibers are often bundled together.
Considerable effort has been made in improving methods for switching data or other signals. It is often necessary to switch data between optical fibers so that it travels correctly from its source to the proper destination. One way to switch data is to electronically detect the optical signals, and electronically switch those electronically-detected signals. New optical signals are then generated from the electronically switched signals, and are sent to appropriate output fibers. This hybrid optical-electrical-optical procedure is relatively complex, requires many optical and electronic components, and does not fully use the available optical bandwidth.
Alternatively, it is possible to switch signals solely in the optical domain, i.e., switching all of the optical signals in a fiber to the appropriate destination fiber with no intervening conversion to an electrical signal. All-optical switching should ideally be simpler, more reliable, and result in better use of bandwidth.
Prior optical crossbars, in which light propagates through integrated optical systems, are described for example in U.S. Pat. Nos. 4,013,000, 5,255,332, and 5,283,844. Prior optical crossbar switching systems have been rather complex, and have not yet fully proven themselves in practice. In the crossbar of the U.S. Pat. No. 4,013,000 patent, grating couplers selectively coupling inputs to outputs are mechanically moved between different positions by piezoelectric drivers; alternatives for mechanically moving the couplers were said to include inducing gratings by electro-optical, acousto-optical, or magneto-optical means. In the crossbar of the U.S. Pat. No. 5,255,332 patent, the reflectivity of grating couplers is altered by voltages applied to the gratings. In the crossbar of the U.S. Pat. No. 5,283,844 patent, redirection of optical signals along an optical waveguide is accomplished by means of total internal reflectance turning mirrors in which a portion of the waveguide, along with surrounding semiconductor layers, are etched to a point beneath a single quantum well and confinement layers, such that optical signals arriving at the etched facet are redirected.
U.S. Pat. No. 5,013,140 describes an N-to-1 optical switch using N deflection stages with polarization rotators and deflection means, e.g. a birefringent calcite crystal or a polarizing beam splitter, to determine which of 2N inputs is selected at the output.
U.S. Pat. No. 4,461,543 describes an optical switch in which a birefringent crystal followed by a polarization rotator is used to cause arbitrarily polarized light to form two beams of the same polarization, which are then deflected to selected parallel paths. A second polarization rotator reestablishes the initial polarization, and the beams are then recombined by a second birefringent crystal.
U.S. Pat. No. 6,031,658 discloses an optical scanner using a binary optical polarization sensitive cascaded architecture having binary switchable optical plates for scanning in one, two, or three dimensions. It was said that several such scanners could be combined on a three-dimensional surface to form an Nxc3x97N fiber optic switch. Each scanner apparently uses a separate lens system for correct coupling to output fibers on another three-dimensional surface. Each scanner relies on an on-chip control birefringent mode nematic liquid crystal device or birefringent plate that can be programmed to generate xe2x80x9cany desired optical wavefront.xe2x80x9d
T. Nelson, Digital Light Deflection, Bell System Technical Journal, pp. 821-845 (May 1964) discloses a digital method of deflecting a light beam using n optical modulators and n birefringent crystals to provide 2n possible output beam positions. The thicknesses of successive birefringent crystals decreased by factors of 2 to allow addressing to binary output positions. Because each crystal can displace an input from one channel into a different channel, it would be cumbersome to adapt this device for use as a switch for multiple inputs, since it would be difficult to keep different beams separated from one another after they had entered a common intermediate channel. Thus this device is well-suited for use as a multiplexer or demultiplexer, switching N channels to 1 channel, or 1 channel to N channels. However, it would be awkward to use this device as a crossbar, which is a device that can arbitrarily, simultaneously, and independently switch each of several inputs to any one of several outputs, without interference between different channels.
xe2x80x9cSmall wonder: MEMS will power optical Internet,xe2x80x9d IEEE Computer (July 2000) describes an all-optical switch based on mechanically controllable silicon microstructures and mirrors.
There is a continuing, unfilled need for an efficient, all-optical crossbar, suitable for switching from multiple inputs to multiple outputs, without interference between channels, and with little or no delay in the transmitted signals.
We have discovered an efficient, all-optical crossbar, in which multiple input sources are optically connected, in any desired combination (including one-to-one and many-to-one permutations), to multiple output receivers. The system is optically simple, has low insertion loss, and may be used to make connections between large numbers of inputs and outputs. It is well-adapted to switching signals on single and multiple mode optical fibers. The novel all-optical crossbar is well-suited to switching massive amounts of data between arrays of fiber optics. Such high-capacity switching will be useful, for example, in many Internet applications. The signal paths in the novel crossbar are all-optical. Electrical signals (control voltages) are used, but only for beam steering control. The signal is never converted to an electronic form during the switching. There is relatively little loss; the primary loss (at least in certain embodiments of the invention) will normally be the 3 db or so loss that results from a linear polarization of unpolarized input light.
The novel optical crossbar uses a series of deflecting elements, such as prisms or birefringent plates, to direct beams of light emitted from an input array of light sources. Each deflector deflects the light in a direction that is a function of the polarization of the light as it passes through the deflector. The deflectors deflect the light through a series of angles, so that after passing through n deflectors the light may be deflected into one of 2n different angles.
The particular output to which each particular input is directed is controlled by voltages applied to several deflection units, where each deflection unit comprises an array of polarization control elements and a polarization-sensitive angular deflector, such as a polarization-sensitive deflection prism. Depending on the output address selected for a given input, a polarization control element may leave the polarization of the light unchanged, or it may rotate the plane of polarization to select a different direction of deflection in the subsequent polarization-sensitive deflector. The various polarization-sensitive deflectors preferably differ in strength (e.g., have differing thicknesses) to allow switching that corresponds to arbitrary binary addressing. Also, the deflection units preferably can deflect light in either of two directions, for example either horizontally or vertically, so that the input and output arrays may be either one-dimensional or (preferred) two-dimensional.
As an example of the geometrical optics that may be used in embodiments of this invention, consider the arrangement illustrated in FIG. 8, in which light is emitted parallel to the optical axis from eight inputs in a linear array 10. In the absence of any deflection, all rays are brought to a point 36 on the optical axis in the back focal plane of the first lens 62 and within the second lens. An exemplary ray 37 is shown from the first input to this point. Similarly, all rays deflected downwards by the first deflection unit 42 are brought to the point 38 in the back focal plane. An exemplary ray 39 is shown from the second input to the point 38. More generally, after passing through three deflection units 42 rays may be directed to any of the eight locations in the back focal plane. A second lens 62 and a second set of deflection units 42 then redirects the light to linear output array 70 so that the light is parallel to the optical axis.
Very similar considerations apply for narrow Gaussian beams, for which physical optics instead of geometric optics are used to describe the propagation of the light.
Note that the dimensions of the polarization control elements and of the polarization-sensitive deflector should be chosen so that, even when a signal experiences the maximum deflection angle, the signal stays within a single channel through the entire control array, so that different input signals are not mixed together. This novel switching of signals is possible because the differential direction used in the present invention is that which transmits the light through different selected angles. By contrast, such differential direction would be more difficult to implement with a calcite birefringent crystal that transmits the light through different parallel displacements that depend on polarization. In the latter case, multiplexing or demultiplexing (N-to-1 or 1-to-N) is possible, but a crossbar switch (N-to-N or N-to-M) may not be practical.
In a preferred embodiment, the light from all of the input sources is initially polarized in the same direction. The direction of polarization of each beam of light is individually controlled at each deflector it encounters to attain the desired deflection angle. This polarization control may be achieved, for example, with an electrically controlled device, such as a liquid crystal cell, or a Pockels or Kerr cell.
After deflection through the series of polarization-sensitive deflectors, the light is imaged onto an output array of receivers, preferably with a single lens. The receivers may, for example, be single- or multiple-mode optical fibers. A second series of polarization-sensitive deflectors and individual polarization controllers, and a second single lens may optionally be used to direct the light to the appropriate output receiver so that its chief ray is parallel to the receiver. This last step optimizes coupling to the receivers, especially in the case of single mode fibers, to minimize power loss.
The input array and output array will ordinarily have the same number of fibers, so that switching is N-to-N. However, the present invention is also well-suited for N-to-M switching where such switching may be desirable (i.e, when Nxe2x89xa0M).
The crossbar will, generally, be insensitive to the wavelength of the light used. The crossbar preserves the wavelength of the light unaltered. The novel crossbar may thus be used in an all-optical router in which signals are first demultiplexed into separate fibers, then routed using the crossbar, and finally re-multiplexed into another set of fibers. This approach is well-suited to add-drop operations in which a selected subset of channels is replaced by a new set of channels.